1. Field
This disclosure relates generally to a wireless communication system and, more specifically, to techniques for generating and detecting a physical random access channel signal in a wireless communication system.
2. Related Art
A physical random access channel (PRACH) is a contention-based channel that has been implemented in various wireless communication systems for initial uplink (UL) transmission. In general, a particular PRACH implementation is dependent on the technology employed by an associated wireless communication system. For example, depending on the wireless communication system, a PRACH can be used to access a network, request resources, carry control information, adjust a time offset of a UL, and/or adjust transmitted power. As the PRACH is a common channel, the PRACH may experience collisions when different user equipment (UE) attempts to simultaneously utilize the PRACH. In order to help prevent a collision on a message of interest, a system may employ a preamble (which is a short signal that is typically sent prior to a transmission of an associated message) for a PRACH access. That is, a PRACH access may include a transmission of a preamble (that is selected from a set of preambles) and a subsequent transmission of an associated message. In a long-term evolution (LTE) compliant wireless communication system, a PRACH only includes a preamble.
In at least one wireless communication system, a UE may persist in sending a preamble (at least for a predetermined number of times) until the UE receives an acquisition indicator (AI) or a random access response message from a serving base station (BS) that indicates the BS correctly detected the preamble. When a positive AI or a positive random access response message is received by a UE, a subsequent transmission of an associated message is contention free, except where multiple UEs have transmitted the same PRACH signal substantially simultaneously (in which case collision resolution is needed). In a typical system, a UE is informed, via a broadcast channel (BCH), which access slots the UE can use for a PRACH. Typically, before a PRACH access, downlink (DL) power is measured (e.g., from the BCH) and an initial transmit power is computed from the measurement. In a typical wireless communication system, the preamble does not include the identity of a transmitting UE. If a BS successfully detects the preamble, the BS sends back a random access response message that includes a replica of the preamble, an indication, and resources reserved for uplink (UL) transmission if the indication is positive.
A high-speed PRACH in an LTE compliant wireless communication system employs a relatively complicated waveform. An LTE PRACH occupies seventy-two tones at 15 kHz in the frequency-domain and a time-period in the time-domain that is based on a format of the PRACH signal. For example, an LTE PRACH signal that employs format ‘0’ has a time duration of about 0.8 milliseconds. Depending on the formats employed, many PRACH waveforms may be possible. In general, PRACH waveform generation at a UE and PRACH waveform detection at a serving BS has conventionally been highly complex.
With reference to FIG. 1, a relevant portion of a conventional receiver 100, which is included within a serving BS, is illustrated. The receiver 100 receives multiple baseband (BB) signals (i.e., via antennas 1 to M). For a 10 MHz system, a BB signal from antenna 1 may be processed using a 12288-point fast Fourier transform (FFT) block 102, which filters the BB signal. The filtered received signal (which may include a PRACH signal) is then provided to an extraction block 104 that extracts tones (e.g., 839 tones) for the PRACH signal, when the PRACH signal is present in the filtered signal. The extracted PRACH tones are then processed by an inverse discrete Fourier transform (IDFT) block 106, which performs an appropriate sized (e.g., an 839-point) IDFT on the extracted PRACH tones to provide a time-domain signal.
A demasking block 108 demasks the time-domain signal with a demasking signal (Xu(n)), i.e., multiplies the time-domain signal with the demasking signal. A discrete Fourier transform (DFT) block 110 then performs an appropriate sized DFT (e.g., an 839-point DFT) on the demasked signal to provide a frequency-domain signal to facilitate time correlation. The frequency-domain signal is then provided to a power combine block 112 that accumulates power for each PRACH signal location of interest. For example, the block 112 may combine power from three peaks associated with the PRACH signal. Next, a noise variance estimation block 114 performs a noise variance estimation based on a predetermined number (e.g., fifty) of the PRACH signal locations with the lowest power.
The noise variance estimation is provided to a threshold block 118, which sets a detection threshold based on the noise variance estimation. Similarly, a peak power selection block 116 performs a peak power selection from a predetermined number (e.g., fourteen) of the PRACH signal locations with the highest power. The peak power and the detection threshold are provided to a threshold and time delay estimation block 120, which provides an indication of whether a PRACH signal detection has occurred. In the event that a PRACH signal detection has occurred, an associated BS may transmit a preamble associated with the PRACH signal in an acquisition indicator (AI) signal along with a positive or negative acknowledgement (based on capacity, etc.).
Similarly, a BB signal from antenna M may be processed using a 12288-point FFT block 122, which filters the BB signal. The filtered signal (which may include a PRACH signal) is then provided to an extraction block 124 that extracts tones (e.g., 839 tones) for the PRACH signal, when the PRACH signal is present in the filtered signal. The extracted PRACH tones are then processed by an IDFT block 126, which performs an appropriate sized (e.g., an 839-point) IDFT on the extracted PRACH tones to provide a time-domain signal. A demasking block 128 demasks the time-domain signal with a demasking signal (Xu(n)) i.e., multiplies the time-domain signal with the demasking signal. A DFT block 130 then performs an appropriate sized DFT (e.g., an 839-point DFT) on the demasked signal to provide a frequency-domain signal to facilitate time correlation. The frequency-domain signal is then provided to the power combining block 112, which (as discussed above) accumulates power for each PRACH signal location. Unfortunately, calculating IDFTs and DFTs for relatively large numbers (e.g., relatively large prime numbers) is computationally expensive and, as such, the PRACH signal detection performance of the conventional receiver 100 is far from ideal.